A number of different methods to increase the capacity of optical fibres in a network are known. One of these comprises to utilize wavelength division multiplexing (WDM) in order to improve the utility of bandwidth in the network, which, however, requires provision of devices capable to multiplex and demultiplex transmission channels that are transferred at different, so-called optical carrier wavelengths, in the network.
The publication A new type of tunable demultiplexer using a multileg Mach-Zehnder interferometer, J. -P. Weber et al, Proc. ECIO ""97 EthE5, Stockholm, pages 272-275, 1997, shows an MMIMZI device 1 (multimode interference Mach-Zehnder interferometer) that may be used for cyclic wavelength selective switching.
C. K. Madsen, A multiport frequency band selector with inherently low loss, flat pass bands, and low cross-talk, Photon. Tech. Lett., Vol. 10(12), pages 1766-1768, 1998.
WO 98/39686 and T. Augustsson, Bragg grating assisted MMIMI coupler for wavelength selective switching, Electron. Lett., Vol. 34(25), pages 2416-2418, 1998 describes an MMIMIBg device (Bragg grating assisted multimode interference Michelson interferometer), which offers an entirely individual switching.
Generally, the problems of the above known techniques comprise a long propagation path for the light through the respective device, which gives rise to large losses and instabilities. Further, to tune (phase control) the device relatively high energy consumption is needed. Further, known techniques may give problems with channel crosstalk and need relatively complicated structures.
It is an object of the present invention to provide a device for demultiplexing a wavelength multiplexed optical signal comprising at least a first and a second optical wavelength channel, which exhibits improved performance.
It is in this respect a particular object of the invention to provide such a device that exhibits better filter profiles and a lower channel crosstalk.
It is a further object of the invention to provide a device for demultiplexing, which is more compact than multiplexers known in the art.
Further objects of the invention will be apparent from the description below.
According to a first aspect of the present invention a device is provided comprising:
(1) an MMI coupler, at least of the size 2xc3x972, for receiving a wavelength multiplexed optical signal comprising at least two wavelength channels at an input and for splitting the multiplexed signal into components and imaging these at several of its ports,
(2) Michelson waveguides for receiving and transporting the components imaged at the ports of the MMI coupler, and
(3) reflecting means for reflecting the components received and transported in the Michelson waveguides back towards the ports of the MMI coupler.
The MMI coupler is further arranged for receiving the reflected components and the respective lengths of the Michelson waveguides are adapted to enable said MMI coupler to combine the components so that the first and the second optical wavelength channels are imaged substantially at different outputs of the MMI coupler.
Preferably, the Michelson waveguide lengths are different and the differences of the lengths between them are determined in dependence on the wavelength channel distance.
The device can further comprise one or several phase adjustment means for phase adjustment of the components received and transported in the Michelson waveguides. These phase adjustment elements may comprise trimming elements for permanent fine adjustment of the phase or of phase control elements for active phase control of the phase.
Further, the device is reciprocal in that it works in an equivalent manner for multiplexing in the back direction.
According to a second aspect of the present invention there is thus provided a device for multiplexing at least a first and a second optical wavelength channel comprising:
(1) an MMI coupler, at least of the size 2xc3x972, for receiving the first and the second optical wavelength channel at a respective input and for splitting said channels into components and imaging these at several of its ports,
(2) Michelson waveguides for receiving and transporting the components imaged at the ports of the MMI coupler, and
(3) reflecting means for reflecting the components received and transported in the Michelson waveguides back towards the ports of the MMI coupler.
The MMI coupler is further arranged for receiving the reflected components and the respective lengths of the Michelson waveguides are adjusted so that said MMI coupler combines the components in such a manner that a wavelength multiplexed optical signal comprising said first and said second optical wavelength channel is imaged substantially at a single output of the MMI coupler.
According to a third aspect of the present invention there is provided a device for optical add/drop multiplexing comprising a device of the above kind for demultiplexing and a device of the above kind for multiplexing.
Preferably, the add/drop multiplexing device comprises an input waveguide for inputting an optical multiplexed signal to the demultiplexer, at least one transit waveguide for outputting a demultiplexed channel from the demultiplexer and for transmission, and inputting the same at the multiplexer, at least one waveguide for outputting a further demultiplexed channel from the demultiplexer for dropping (dropping functionality), at least one waveguide for inputting a further channel to the multiplexer (add functionality), and an output waveguide for outputting an optical multiplexed signal comprising the demultiplexed channel transmitted in the transit waveguide and the further signal input to the multiplexer.
In one embodiment of the present invention the demultiplexer and multiplexer of the add/drop multiplexing device have active phase control elements, whereby the device exhibits wavelength selective tunability.
It is a further object of the invention to provide at least one method for demultiplexing a wavelength multiplexed optical signal.
According to a fourth aspect of the present invention there is provided a method for demultiplexing comprising the steps of:
(1) inputting the wavelength multiplexed optical signal at an input of an MMI coupler, at least of the size 2xc3x972, and splitting the signal into components, which are imaged at several ports of the MMI coupler,
(2) inputting and transporting the components imaged at the ports of the MMI coupler in the Michelson waveguides,
(3) reflecting the components input and transported in the Michelson waveguides back towards the ports of the MMI coupler at adapted distances from the MMI coupler, and
(4) inputting and combining the reflected components in the MMI coupler, whereby the first and the second optical wavelength channels are imaged substantially at different outputs.
The advantages of the present invention are principally that the device is compact and thereby admits a short propagation path for the light. Thus, lower losses and stability against temperature gradients are achieved. Further, the invention shows a flexible functionality, provides possibilities for creating improved performance and that it in some respects exhibits more system tolerant properties in comparison with known art, which uses linear phase responses.